Example embodiments relate to small cell and macro cell synchronization.
Mobile radio frequency band(s) are both scarce and precious resources. After the inception of commercial mobile radio communication in the 1980's the numbers of subscribers have been growing exponentially. The underlying radio technology also has grown at a fast pace. In addition to conventional voice communication, data, video and real time gaming have been introduced.
These new services require a relatively higher number of bits transmitted in a unit time than conventional voice services. There are two main ways to achieve larger bit rate demands, first, efficient use of spectrum using advanced technology (based on, for example, multiple transmit and receive antennas) and, second, the use of a larger frequency band. As the frequency spectrum is already crowded the latter is often not feasible.
Introduction of the cellular concept in the 1980's allowed efficient reuse of frequency spectrums. A service area may be divided into hexagonal grids of cells, which are further grouped into clusters of cells. The frequency band may be apportioned within and reused between the clusters so as to intelligently keep the co-channel interference low.
Next generation wireless technologies are based on code division multiple access (CDMA) technologies that are more robust to interference and thus universal frequency reuse or re-use of the same frequencies across cells was introduced in 2nd and 3rd generation CDMA networks.
Orthogonal frequency division multiplexing (OFDM) technology is the technique used for future 4G or International Mobile Telecommunications (IMT)-advanced networks. While OFDM is a spectrally efficient scheme and is also more suitable for multiple antenna techniques (MIMO), OFDM is more susceptible to interference. Therefore, the efficient and intelligent use of the frequency spectrum across cells is important for successful deployment of the OFDM networks.
Substantial research effort has been devoted to improve spectral efficiency, or in other words, frequency reuse of the OFDM system. Several solutions have been proposed, e.g., fractional frequency reuse (FFR) (dynamic and static), inter-cell interference coordination (ICIC) and small cell deployment (heterogeneous networks).
FFR uses a portion of the spectrum for a certain area of the cell. The portion of the spectrum is dynamically changed or allocated in a static manner. If the spectrum is dynamically allocated the uplink control signals from the surrounding cells may be used to make the allocation decisions.
In ICIC the cells periodically share some metric, for example a channel quality indicator (CQI), of a frequency band via the backhaul communication interface. A cell makes the decision to allocate a frequency band from its own measurements and the information received from the surrounding cells.
Small cell deployments within a larger macro cell efficiently use the spectrum and deliver the demand for the higher bit rate in certain areas of the cell. Generally the small cells use lower transmit power to serve a small area where the demand for the service is high, or in other words, they have a cell radius of a few meters to a few hundred meters. Small cells may use wireless or wired backhaul connections to the back bone network.
Indoor and outdoor pico cells, femto cells and micro cells are the main types of small cells. The categorization of the small cells are based on, for example, their transmit power levels, deployment scenarios and/or the ownership of the small cell network. If different types of small cells are deployed within a macro cell the network is also called a heterogeneous network.
FIG. 1 illustrates a conventional heterogeneous network 100. As shown, a plurality of cells 105 are arranged in a hexagonal grid of cells. Each cell may include one or more antennas 115 associated with, for example, a base station (not shown). One or more of the cells may include a plurality of small cells 115 to support services in a localized area within a cell 105. An enhanced nodeB (eNB) 110 serves the plurality of cells 105.
The widely used GSM, GPRS, UMTS, HSDPA and HSUPA wireless macro cellular standards were created by the third generation partnership project (3GPP). 3GPP recently finalized the LTE standard (Release 8) and is working towards their new standards namely, releases 9 and 10. Release 10 is targeted to satisfy the IMT-advanced specifications. Currently several operators around the world are planning to deploy LTE technology for their future cellular network with macro cells, pico cells and femto cells to deliver the demand for the higher data rates.
In a heterogeneous network such as FIG. 1, macro coverage is overlapped with spotty small cell coverage at commercial areas or residential areas. Most of the small cells, such as femto cells or even public pico cells, are deployed in indoor environments. Thus, there is difficulty for indoor small cells to be synchronized with the umbrella macro cells. A common synchronization method is based on GPS. However, indoor small cells have difficulty in acquiring the GPS signals. The measured strength of the GPS signals indoor is 30 dB lower than the measured strength of the GPS signals outdoors. Furthermore, due to a cost constraint, many small cells do not have a GPS receiver.